The present invention relates to an apparatus for recording and/or reproducing a serial digital signal, and is directed more particularly to an apparatus for recording and/or reproducing a pulse-code-modulated audio signal on a moving recording medium.
An analog signal, such as an audio signal, may be repetitively digitized at a sampling frequency. Each time the analog signal is sampled, a multi-bit digital representation of the amplitude thereof is produced. For example, a digital number of 8 bits may be produced for each sample of the audio signal. The 8 bits may be serially recorded on a recording medium, for example, in a track extending longitudinally on a tape, in parallel oblique tracks on a video tape, or on a video disc, for later reproduction.
When a signal is recorded by a magnetic head on a moving magnetic medium, a given frequency can be considered to produce a wavelength on the magnetic recording medium which depends on the transport speed of the magnetic medium.
Magnetic recording and reproducing heads depend upon a recording gap therein for producing or detecting a magnetic field in the tape or other recording medium. When the wavelength of the highest frequency signal is large compared to the gap width, the reproduced output is degraded. Alternatively, when the wavelength of the maximum frequency is reduced to less than twice the gap width, a rapid loss in reproduced signal is obtained. It is, therefore, desirable to maintain a transport speed for the record medium which is high enough to give good output but not so high as to produce too short a wavelength.
In addition to the above factors, it is desirable to produce as short a wavelength as possible on the recording medium in order to have a maximum recording density. This permits the use of a minimum amount of recording medium for a given amount of recorded information. That is, if a low sampling frequency is employed, a correspondingly low transport speed may be used in order to produce an optimum data density on the recording medium. Conversely, when a high sampling frequency is used, a high transport speed may be used to produce the same data density.
The above-mentioned change in the transport speed of the recording medium becomes important when a number of different sampling frequencies and/or data sources must be accommodated. For example, pulse code modulation signal recording and/or reproducing may be effected with apparatus of the rotary head type using a video tape recorder and with apparatus of the stationary head type using a linear tape drive.
In the rotary head video tape recorded system, a pulse code modulated signal is time-base-compressed to provide data-missing periods into which horizontal and vertical synchronizing signals of a standard television signal may be inserted for controlling the video tape recorder on playback. In order to provide a sampling frequency high enough to record and reproduce an audio signal having a bandwidth of 20 KHz, and to provide an integral number of samples during each horizontal period, a sampling frequency of at least 40 KHz is required. A sampling frequency of 44.056 HKz has been employed for recording on a rotary head video tape recorder which is normally employed to process signals of the NTSC color system. This sampling frequency permits recording of three samples per horizontal interval and accommodates 35 non-recordable lines in each NTSC frame for receiving the equivalent of the synchronizing signals.
In a stationary head system, a sampling frequency is selected to again permit recording and reproduction of an audio signal having a bandwidth of 20 KHz and desirably which has an integral relationship with the above sampling frequency for a rotary head system. For this reason, a sampling frequency for a fixed head system may be 50.35 KHz for compatibility with the above-described NTSC rotary head system. Such sampling frequencies for the rotary head and fixed head systems are related by a factor of 8/7.
Other sampling frequencies have been proposed such as 32 KHz, which is compatible with the data rate in the microwave links of the British Broadcasting Corporation, and 48 KHz in other recording devices. A sampling frequency of 32 KHz is not, of course, capable of providing a bandwidth of 20 KHz.
Additional problems occur in a fixed head system having separate record and playback heads when editing is attempted by cut-in of new recorded material on a track previously recorded using a different sampling frequency. Such apparatus normally employs a playback head spaced a significant distance forward of the record head. The playback head reproduces the previously recorded information which then is processed to reconstruct the original signal. Similarly, new information to be recorded must be prepared by sampling, converting and formatting before being applied to the recording medium. Both reconstruction of previously recorded information and preparation of new information take a relatively long time. A relatively large spacing is, therefore, needed between the playback and record heads to accommodate this delay for a particular recorded data density. If the new material to be recorded has a sampling rate which is different from the originally recorded material, then an attempt to joint edit new and old material for cross-fading or the like with a constant transport speed of the tape interrupts the continuity of the original signal and a sound skip may result at the cut-in point.